1. Field of the Invention
The present invention relates to a birefringent film employable for a liquid crystal display, and a liquid crystal display provided with the birefringent film.
2. Description of Prior Art
A liquid crystal display can be directly joined to a IC circuit to be driven at low electric voltage and power, and further has various displaying functions and possibilities of making its weight lighter and making its productivity higher. As a result, the use of the liquid crystal display has developed.
A super twisted nematic liquid crystal display (hereinafter referred to as STN-LCD) has been recently utilized for dot-matrix type liquid crystal displays such as those of word processors and personal computers. The liquid crystal display generally has a structure that a liquid crystal cell is disposed between a pair of polarizing sheets. The STN-LCD uses a super twisted nematic liquid crystal showing a twisted angle of not less than 160 degrees. Such STN-LCD has an advantage of showing a high contrast on high multiplexing drive, compared with the conventional twisted nematic liquid crystal display (twisted angle: 90 degrees) which can be utilized for the dot-matrix type liquid crystal displays.
A display screen of the STN-LCD shows an image inherently colored with the hue from green to yellowish red owing to elliptically polarized light transmitted through STN liquid crystal cell. This phenomenon may be hereinafter referred to as coloring. In order to avoid the coloring of the display screen, there have been proposed a liquid crystal display having one or three layers of optically anisotropic body (that is, NTN mode-display) and a liquid crystal display using a uniaxial stretched polymer film (that is, FTN mode-display).
In the NTN mode-display, a linearly polarized light transmitting through one side of a pair of polarizing sheets is changed to the elliptically polarized light almost aligned in the long-axis in the wavelength range of about 400 to 700 nm. Further, the elliptically polarized light is transmitted through the other polarizing sheet, to show a white light without cutting of any wavelength range.
The FTN mode-display utilizes a uniaxial stretched polymer film serving as optically anisotropic body instead of the liquid crystal cell. The former NTN mode-display shows excellent characteristics in black-and-white display. The liquid crystal cell for optically anisotropic body used in the NTN mode-display use is voluminous and a plurality of such cells are required, so that the NTN mode-display is larger in volume and weight, and higher in production cost than the FTN mode-display.
Japanese Patent Provisional Publication No. 63(1988)-189804 proposes, as a uniaxial stretched film for the FTN mode, use of a polycarbonate film which is uniaxially stretched so that a value of retardation measured by a polarizing microscope is set in the range of 200 to 350 nm or 475 to 625 nm. The retardation is defined by the product of birefringence (.DELTA.n) and thickness (d) of the film.
Japanese Patent Provisional Publication No. 63(1988)-167304 discloses a laminated film in which two or more uniaxial stretched films having birefringence are so laminated as to allow their main optic axes to intersect at right angles each other. In the case that two birefringent films (their retardation values are named R.sub.1 and R.sub.2, respectively) are laminated under the intersection at right angles, the retardation of the composite film (i.e., phase difference film) shows a retardation value of ".vertline.R.sub.1 -R.sub.2 .vertline.". Such film can be adjusted so that the value of the ".vertline.R.sub.1 -R.sub.2 .vertline." is set in the range of 90 to 180 nm, 200 to 350 nm or 475 to 625 nm. Thus, the composite film prevents the display screen from coloring.
The above polymer films (birefringence films (phase difference films)) have been developed for the purpose of the elimination of coloring of the STN-LCD. Thus, the coloring is reduced and the display provided with the film shows almost a black-and-white image. Further, the liquid crystal display utilizing the FTN mode has the advantages of lightweight and low production cost compared with the NTN mode-display as mentioned previously.
As described above, the above liquid crystal display has been improved in elimination of coloring, so far as the coloring view from the direction vertical to the screen is concerned. However, when the liquid crystal display is viewed from an oblique direction, unfavorable viewing angle characteristics such as coloring and disappearance of displayed image are observed.
In more detail, a conventional phase difference film using a polymer having a positive birefringence is adjusted under the following condition: EQU n.sub.MD &gt;n.sub.TD .gtoreq.n.sub.ND
in which "n.sub.MD " is a refractive index in a stretched axis direction of a uniaxially stretched film made of a polymer having a positive intrinsic birefringence, "n.sub.TD " is a fractive index in a direction intersected at right angles to the stretched axis of the film and "N.sub.ND " is a refractive index in a normal line direction to the surface of the film as shown in FIG. 3.
Hence, in the case that a light is incident perpendicular to the surface of the film, Re (retardation) is represented by the following formula: EQU Re=(n.sub.MD -n.sub.TD)d
wherein "d" is a thickness of the film. In the case that a light is incident intersected at right angles to the stretched direction of the film, a birefringence varies between ".DELTA.n=n.sub.MD -n.sub.TD " and ".DELTA.n=n.sub.MD -n.sub.ND " with variation of incident angles, where the following relationship: EQU n.sub.MD -n.sub.TD .ltoreq.n.sub.MD -n.sub.ND
is satisfied. Therefore, .DELTA.n tends to increase (or not to change) with variation of the angles of oblique incidence. On the other hand, an optical path becomes larger due to the oblique incidence, so that "Re (=.DELTA.nd)" also rapidly increases with increase of the angle of the oblique incidence. Further, in the case that a light is incident with tilting to a normal line direction of the film to the stretched direction, .DELTA.n decreases rapidly from "n.sub.MD -n.sub.ND " to "n.sub.ND -n.sub.TD " so that even increase of an optical path can not compensate decrease of the .DELTA.n, whereby "Re (=.DELTA.nd)" is rapidly decreased with increase of the angle of the oblique incidence. In even uniaxially stretched film which is in principle considered that variation of the retardation (Re) is the least (i.e., in the case of "n.sub.MD &gt;n.sub.TD =n.sub.ND "), the Re varies greatly with increase of optical path accompanied by the oblique incidence.
In order to improve the viewing angle characteristics, various studies have been made from the viewpoint of variation of refractive indices in three-dimension directions of the film.
Japanese Patent Provisional Publication No. 2(1990)-256023 discloses as follows. On the basis of findings that the visual angle dependence of the retardation (defined by the product of birefringence (.DELTA.n) and thickness (d) of the film) and the viewing angle of the LCD are closely concerned with each other, the viewing angle characteristics are greatly improved by the use of a film essentially having an optical axis in a normal line direction to the surface of the film, for example, by inserting a laminated film comprising a biaxially stretched film of a polymer having a negative birefringence and a uniaxially stretched film of a polymer having a positive birefringence between a liquid crystal cell and a polarizing sheet.
Japanese Patent Provisional Publication No. 3(1991)-206422 discloses that the viewing angle characteristics are greatly improved by inserting a laminated film comprising a uniaxially stretched film of a polymer having a negative birefringence and a uniaxially stretched film of a polymer having a positive birefringence between a liquid crystal cell and a polarizing sheet.
Japanese Patent Provisional Publication No. 3(1991)-24502 also discloses improvement of the viewing angle characteristics by the use of a laminated film comprising a film of polymer having a negative birefringence and a film of a polymer having a positive intrinsic birefringence. In the above Publications, a polymer used in a film having a negative birefringence is a homopolymer derived from styrene or a copolymer derived from styrene and other monomer(s).
The present inventors have studied to improve the viewing angle characteristics and heat resistance of the copolymer containing a styrene unit from the viewpoint of a structure of the copolymer, copolymerization ratio, or stretching conditions of the copolymer. As a result, phase difference films improved in the above characteristics have been obtained (U.S. Pat. No. 5,213,852).
However, the above-mentioned homopolymers and copolymers derived from styrene (or other monomer) are all relatively brittle, so that the polymers are apt to be ruptured during uniaxial stretching or to be broken during processing such as cutting or punching and further a film of the polymer after incorporated into a liquid crystal display is apt to broken upon receiving variation of temperature.